Recently as a battery pack used for a rechargeable load device having relatively high power consumption such as an electrical power tool, a battery pack using lithium-ion batteries has attracted attention because it has high energy density and is lightweight. However, such a lithium-ion battery is easily damaged by overcharge or over discharge. Hence, generally, in view of improvement of reliability and safety, a protective circuit for monitoring each cell voltage is provided in the battery pack, and charge-discharge control is performed based on the cell voltage.
Generally, the aforementioned protective circuit is constituted by an IC designed for this use. When the protective circuit is in operation even when the battery pack is not connected to the charger or even when a load device powered by the battery pack as its power source is turned off, a remaining amount of the battery may be reduced due to a current consumed by the protective circuit.
In view of the above, for example, document 1 (JP 2008-199827 A) discloses a battery pack which switches the protective circuit to a rest state or a standby state with a low consumption current when the battery pack is not connected to the charger or when the load device connected to the battery packs is turned off. Further, this battery pack switches the protective circuit to an operating state in response to an activation signal inputted from the charger or the load device when the battery pack is connected to the charger or when the load device is turned on.
Normally, the activation signal is sent from the charger. However, when the breakage of the charger or the contact failure of a connector for transferring the activation signal occurs, the activation signal is not inputted into the battery pack. In this situation, the charging process is started while the protective circuit does not perform the protective operation. Thus, the battery pack is likely to be in an overcharged state.